Esophagus

Development of Esophagus

The esophagus is the first section of the foregut and begins at the distal end of the pharynx. As with the rest of the digestive tract, the cells that line the lumen of the esophagus are derived from endoderm. The supporting structures of the esophagus come from two different sources, although they are all innervated by the vagus nerve. The muscles and connective tissues of the esophagus's proximal third are derived from the mesenchyme of the pharyngeal arches. Like the pharynx, the skeletal muscle of the superior esophagus is innervated by axons from the nucleus ambiguus traveling in the vagus nerve. The muscles and connective tissues of the distal third of the esophagus are derived from the visceral mesoderm that surrounds the gut tube. For this reason, the muscular layers of this area are composed of smooth muscle, innervated largely by the dorsal vagal motor nucleus, also traveling in the vagus nerve. The middle third of the esophagus blends the characteristics of the other two, containing both skeletal and smooth muscle.

The development of the esophagus is intimately related to that of the trachea. At the distal end of the pharynx during the fourth week, the laryngotracheal groove forms, leading to a short blind pouch, the respiratory diverticulum. The respiratory diverticulum is a pouch of endoderm that extends ventrally into the nearby visceral mesoderm. By the fifth week the respiratory diverticulum has elongated into a tracheal bud that stretches inferiorly and is completely separate from the esophagus except for its connection at the primordial laryngeal inlet, which will eventually become the glottis. The separation of the two tubes is effected by two ridges of visceral mesoderm, the tracheoesophageal ridges. These ridges grow medially between the respiratory diverticulum/tracheal bud and the esophagus, eventually forming a tracheoesophageal septum. This pro­cess occasionally goes awry, resulting in congenital anomalies such as blind pouches and tracheoesophageal fistulas.

The esophagus is initially relatively short but elongates to its normal relative length by the seventh week. If it fails to lengthen appropriately, it can produce a congenital hiatal hernia from the traction placed on the developing stomach as it passes through the diaphragm. The proliferation of the endodermal cells of the esophageal lumen is one of the factors that allow it to lengthen. The lumen typically becomes obstructed by these epithelial cells but recanalization opens the lumen during the eighth week. Failure of the lumen to recanalize may lead to an abnormal narrowing, esophageal stenosis, as well as polyhydramnios, because the fetus is unable to swallow amniotic fluid.

Topographic Relationships; Contours and Normal Constrictions of Esophagus

The esophagus begins in the neck as a continuation of the pharynx (cervical esophagus). This point of origin corresponds to the inferior border of the cricoid cartilage and the lower margin of the inferior pharyngeal constrictor muscle, also called the cricopharyngeus muscle, at about the level of the sixth cervical vertebra. The esophagus extends inferiorly through the neck and through the superior and posterior mediastina of the thorax. It then passes through the esophageal hiatus of the diaphragm to join the cardiac region of the stomach at about the level of the 10th thoracic vertebra.

The esophagus generally follows the anteroposterior curvature of the vertebral column, except in the inferior portion, which is tethered by its relationship with the diaphragm. It also forms two lateral curvatures, so that in a coronal view, it assumes the form of a gentle reversed “S.” The upper of the two lateral curvatures is convex toward the left, and the lower curvature, in the lower thorax and abdomen, is convex toward the right. From its commencement at the lower margin of the cricoid cartilage, the esophagus inclines slightly to the left until its left border projects approximately one-fourth inch to the left of the tracheal margin. It then swings somewhat to the right, reaching the midline at about the level of the fourth thoracic vertebra behind the aortic arch. It continues its inclination to the right until about the level of the seventh thoracic vertebra, where it again turns left somewhat more sharply than in its previous curves, and in this direction it passes through the esophageal hiatus.

The esophagus has cervical, thoracic, and abdominal portions. Anterior to the cervical portion lies the membranous posterior wall of the trachea, to which it is rather loosely connected by loose connective tissue and some smooth muscular strands, so that the anterior esophageal and the posterior tracheal walls are occasionally referred to as the “common party wall.” In the grooves on each side between the trachea and the esophagus are the recurrent laryngeal nerves, which ascend from the vagus nerves in the upper thorax to reach the larynx. Posteriorly, the esophagus lies upon the prevertebral fascia, covering the anterior surface of the longus colli muscles and cervical vertebral bodies. On the left and right, the carotid sheath and the structures it contains (vagus nerve, carotid arteries, and internal jugular vein) accompany the cervical esophagus. Owing to the curvature of the esophagus in this region, it lies closest to the left carotid sheath. The lobes of the thyroid gland partially overlap the esophagus on each side. The thoracic duct ascends in the root of the neck on the left side of the esophagus and then arches laterally posterior to the carotid sheath and anterior to the vertebral artery and vein to enter the left brachiocephalic or left subclavian vein at the medial margin of the anterior scalene muscle.

The thoracic esophagus also lies posterior to the trachea as far as the level of the fifth thoracic vertebral body, at which point the trachea bifurcates. The trachea deviates slightly to the right at its lower end, so that the left main bronchus crosses anterior to the esophagus. Below this point the esophagus is separated anteriorly from the left atrium of the heart by the pericardium. In the very lowest portion of its thoracic course, the esophagus passes posterior to the central tendon of the diaphragm to reach the esophageal hiatus. On the left side in the superior thoracic region, the esophageal wall contacts the ascending portion of the left subclavian artery and the parietal pleura; at about the level of the fourth thoracic vertebra, the arch of the aorta passes posteriorly alongside the esophagus. Below this point the descending aorta lies to the left, but when it passes posterior to the esophagus, the left mediastinal pleura again comes to abut on the esophageal wall. On the right side the right parietal pleura is intimately applied to the esophagus, except when, at about the level of the fourth thoracic vertebra, the azygos vein intervenes as it turns anteriorly. The short abdominal portion of the esophagus lies upon the diaphragm and makes an impression on the liver with its anterior aspect.

The left and right vagus nerves associate with the esophagus, and below the tracheal bifurcation, they interweave to form the esophageal plexus of nerves. This plexus then coalesces with the anterior and posterior vagal trunks that pierce the diaphragm along the esophagus.

The course of the esophagus is marked by several indentations and constrictions:

• 1.

  The first narrowing of the esophagus is found at its commencement, caused by the cricopharyngeus muscle at the inferior border of the inferior pharyngeal constrictor and the cricoid cartilage.

• 2.

  The esophagus is indented on its left side by the arch of the aorta (aortic constriction), and at this level the aortic pulsations may often be observed through the esophagoscope.

• 3.

  Just inferior to this point the left main bronchus causes an impression on the left anterior aspect of the esophagus.

• 4.

  At its lower end the esophagus is narrowed by the diaphragmatic constriction (inferior esophageal sphincter) as it passes through the right diaphragmatic crus.

The overall length of the esophagus varies in accordance with the length of the trunk of the individual. The average distance of the cardia from the upper incisor teeth is approximately 40 cm, but in some “long” individuals this distance may be as much as 42 or 43 cm. This average distance of 40 cm may be subdivided as follows: The distance from the incisor teeth to the cricopharyngeus muscle, which corresponds to the commencement of the esophagus, averages 16 cm. It is thus apparent that the average length of the esophagus itself is 40 minus 16 cm equals 24 cm, or approximately 10 inches. At about 23 cm from the incisor teeth, the arch of the aorta crosses the esophagus on its left side. This crossing is therefore about 7 cm below the cricopharyngeus. A few centimeters below this point the left main bronchus passes anterior to the esophagus. The diaphragmatic constriction, or commencement of the abdominal part of the esophagus, is located at about 37 to 38 cm from the incisor teeth. It is of considerable significance to note that the esophageal hiatus of the diaphragm is slightly inferior (≈ 1 cm) to this point, and the cardiac region of the stomach is at a still slightly lower level. The figures given above are for adults; in children the dimensions are proportionately smaller. At birth the distance from the incisor teeth to the cardiac region is usually only 18 cm, at 3 years approximately 22 cm, and at 10 years approximately 27 cm.

Although the esophagus is usually described as tubelike, it is generally flattened so that the transverse axis is somewhat larger than the anteroposterior axis. In the resting state the esophageal walls are in approximation. The width or diameter of the esophagus varies considerably with its state of tonus, but the average resting width is approximately 2 cm.

Musculature of Esophagus

The musculature of the esophagus consists of an outer longitudinal muscle layer and an inner muscular layer, generally described for convenience as the circular muscle layer, although, strictly speaking, the term “circular” is not properly accurate, as will be seen below. The outer longitudinal muscle layer originates principally from a stout tendinous band that is attached to the upper part of the vertical ridge on the dorsal aspect of the cricoid cartilage. From this tendon two muscle bands originate and diverge as they descend and sweep around the right and left sides of the esophagus. They meet and interdigitate somewhat in the posterior midline, leaving a V-shaped gap superior to and between them. This gap is known as the V-shaped area (of Laimer), and the base of the area is formed by the underlying circular muscle. Superiorly, it is bounded by the cricopharyngeus muscle. A few sparse fibers of the longitudinal muscle spread over this area, as do some accessory fibers from the lower margin of the cricopharyngeus. The longitudinal muscle fibers are not uniformly distributed as they descend over the surface of the upper esophagus. Instead, the fibers gather into thick lateral longitudinal muscle masses on each side of the esophagus, but they remain considerably thinner over other parts of the tube. The muscle is thinnest on the anterior wall (i.e., the wall that is applied to the posterior surface of the trachea). Indeed, high up on the esophagus's anterior surface, the longitudinal muscle is said to be entirely lacking, and this portion of the esophagus is designated as the “bare” area. The longitudinal muscle of the esophagus also usually receives additional contributions by way of accessory muscle slips on each side, which originate from the posterolateral aspect of the cricoid cartilage and also from the contralateral side of the deep portion of the cricopharyngeus muscle. As the longitudinal muscle descends, it progressively forms a more uniform sheath over the entire circumference of the esophagus. The anterior wall of the esophagus is firmly applied to the posterior tendinous wall of the trachea in its upper portion where the two organs are attached to each other by fibroelastic membranous tissue containing some muscle fibers.

The inner, so-called circular layer of esophageal muscle underlies the longitudinal muscle layer. Although a definite layer, it is slightly thinner than the longitudinal coat. This ratio of longitudinal and circular muscle coat is unique for the esophagus and is reversed in all other parts of the alimentary tract. The circular layer in the upper esophagus is not truly circular but rather elliptical, with the anterior part of the ellipse at a lower level than the posterior part. The inclination of the ellipses becomes less as the esophagus descends, until, at about the junction of the upper and middle thirds, the fibers run in a truly horizontal plane. Here, for a segment of about 1 cm, they may be said to be truly circular. Below this point they again become elliptical, but the inclination is reversed from that of the higher fibers (i.e., the posterior part of the ellipse now assumes a lower level than the anterior part). In the lower third of the esophagus, the course of the fibers again changes to a screw-shaped or spiral course, winding progressively inferiorly as they pass around the esophagus. It should be noted also that the elliptical, circular, and spiral fibers of this layer are not truly uniform and parallel but may overlap and cross or even have clefts between them. Some fibers in the lower two thirds of the esophagus occasionally leave the elliptical or spiral fibers at one level, to pass diagonally or even perpendicularly upward and downward to join the fibers at another level, but they never form a continuous layer. They may be threadlike or 2 to 3 mm in width and from 1 to 5 cm in length; they are usually branched. The musculature of the esophagogastric junction will be discussed in the next section. Spontaneous rupture of the esophagus almost invariably occurs in the lower 2 cm of the esophagus. A linear tear may occur through the entire thickness of the esophageal wall. Severe vomiting predisposes to rupture of this region, releasing gastric juice into the mediastinum.

The cricopharyngeus muscle, although strictly speaking a muscle of the pharyngeal wall, being the lowermost portion of the inferior constrictor of the pharynx, is nevertheless of great importance in the function and malfunction of the esophagus. This narrow band of muscle fibers originates on each side from the posterolateral margin of the cricoid cartilage and passes slinglike around the posterior aspect of the pharyngoesophageal junction. Its superiormost fibers ascend to join the median raphe of the inferior constrictor muscle posteriorly. The cricopharyngeus also has muscle fibers that run horizontally to encircle the pharyngoesophageal junction, acting as the superior pharyngeal constrictor. This cricopharyngeal constriction is felt when an esophagoscope is introduced, because even at rest the muscular tonus felt within the esophageal lumen is greater at the level of the cricopharyngeus than in other parts of the esophagus, and the relaxation of this muscle is an integral part of the act of swallowing. Superior to the cricopharyngeus (between this muscle and the main part of the inferior constrictor) the musculature is somewhat weaker and sparser posteriorly. It is through this sparse area that most Zenker diverticula are believed to originate.

The musculature of the upper portion of the esophagus is striated, whereas that of the lower portion is made up almost entirely of smooth muscle. The level at which this transition takes place varies. In general, it may be said that the upper fourth of the esophagus contains purely striated muscle, the second fourth is a transitional zone in which both striated and smooth muscle are present, and the lower half contains purely smooth muscle. Between the longitudinal and circular coats of the esophageal muscles is a narrow layer of connective tissue where the myenteric ganglia and plexus (of Auerbach) can be found.

Esophagogastric Junction

The structure and function of the distal esophagus and its junction with the stomach have been the subject of much investigation. This has led to an improved understanding of clinical ailments such as achalasia, hiatal hernia, Barrett esophagus, esophagitis, and peptic ulcer of the esophagus. The longitudinal muscle coat of the esophagus extends inferiorly and continues over the surface of the stomach as the outer longitudinal layer of the smooth muscle of the stomach. The so-called inner circular layer of esophageal musculature, which at this point is spiral in character, also continues over the stomach but divides, in the region of the cardia, into the middle circular layer of the gastric musculature and the inner oblique layer of muscle fibers. The esophagus's inner oblique muscle fibers pass slinglike across the cardiac notch, whereas the middle circular fibers pass more or less horizontally around the stomach. These two layers of muscle fibers thus cross each other at an angle, forming a muscular ring, which became known as the collar of Helvetius. To the left of the esophagus the oblique fibers form sling fibers that project from the anterior wall of the stomach to the posterior wall, making a tight bend around the cardial notch. On the opposite side of the cardiac region, the circular layer has substantial clasp fibers that pinch and narrow the cardiac region. Acting together, the sling and clasp fibers help prevent gastric reflux and form a functional, but not anatomic, lower esophageal sphincter.

A gradual but moderate thickening of both the so-called circular and longitudinal muscles takes place in the lower end of the esophagus, commencing about 1 or 2 cm above the esophageal hiatus through the diaphragm and extending to the cardia. This region of thickened musculature has been termed the “esophagogastric vestibule” and this region contracts and relaxes as a unit. It is believed that the bolus is transiently arrested just above the esophageal hiatus by the tonicity of the distal esophagus and, contrariwise, that its passage into the stomach is made possible by the relaxation of the muscles working as an integrated or coordinated unit. It is likewise believed that the contraction of the distal esophagus is one of the important factors in the prevention of regurgitation from the stomach. Other factors in the prevention of regurgitation are believed to be the angulation of the esophagus as it passes through the diaphragm while passing over into the stomach and a rosette-like formation of loose gastric mucosa at the cardia. The possibility of sphincteric action of the diaphragm is debated, although it is recognized that in deep inspiration, when the diaphragm is in strong contraction, passage into the stomach may be impeded.

The mucosa of the esophagus is smooth and rather pale in color. When the esophagus is contracted, the mucosa is gathered up into irregular longitudinal folds. The gastric mucosa, on the other hand, is a much deeper red in color, with well-defined folds, rugae, in the lumen of the organ. The transition from esophageal to gastric mucosa occurs rather sharply and is easily recognizable by a change in epithelial color. This transition takes place along an irregular dentate or zigzag line, sometimes called the Z line. The Z line marks the transition from stratified squamous of the esophagus to simple columnar epithelium of the stomach; it usually does not coincide with the anatomic border of the cardia but is slightly superior to it, between the level of the cardia and the esophageal hiatus. In some instances the gastric mucosa may extend for a considerable distance into the esophagus.

In its passage through the esophageal hiatus of the diaphragm, the esophagus is surrounded by the phrenicoesophageal ligament, also known as the phrenoesophageal ligament or diaphragmaticoesophageal ligament. The phrenicoesophageal ligament arises from the circumference of the esophageal hiatus as an extension of the infradiaphragmatic fascia, which is continuous with the transversalis fascia. At the margin of the hiatus, it divides into an ascending limb and a descending limb. The ascending limb passes superiorly through the esophageal hiatus and surrounds the esophagus in a tentlike fashion. It extends for several centimeters above the hiatus and inserts circumferentially into the adventitia of the esophagus. The descending limb passes inferiorly and inserts around the cardia deep to the peritoneum. The two limbs of the phrenicoesophageal ligament form a space superficial to the esophagus, in which lies a ring of rather dense fat. The function of the phrenicoesophageal ligament has been the subject of much speculation. From its structure it certainly would seem to play, fixing the distal esophagus in place while permitting the limited excursion required for respiration, deglutition, and postural changes. It also serves as an additional means of preventing pressure transmission through the esophageal hiatus. It may also in some manner take part in the closure or sphincteric mechanism of the esophagus in connection with diaphragmatic action.

The configuration of the esophageal hiatus of the diaphragm is interesting in that the distal esophagus is directed toward the left yet the hiatus is formed almost entirely by the right crus of the diaphragm; the left crus of the diaphragm plays no part in the formation of the esophageal hiatus. One band of muscle fibers, originating from the right crus, ascends and passes to the right of the esophagus. Another band of muscle fibers, originating also from the right crus but more deeply, ascends and passes to the left of the esophagus. These muscle bands overlap scissorwise and are inserted into the central tendon of the diaphragm. Thus, all the muscle fibers about the esophageal hiatus arise from the right crus of the diaphragm. It is interesting to note that those fibers of the right crus which pass to the right of the esophagus are innervated by the right phrenic nerve, whereas those which pass to the left of the esophageal hiatus appear to be innervated by a branch of the left phrenic nerve, as is also the left crus itself. The right crus of the diaphragm is usually considerably larger than is the left crus.

Occasionally, one may find what has come to be known as the “muscle of Low.” This is a small band of muscle fibers that originates from the left crus and crosses over to the right, passing between the muscle fibers of the right crus to reach the central tendon in the region of the foramen of the inferior vena cava. Somewhat more frequently, a similar muscle bundle appears on the superior surface of the diaphragm. More significant is the fact that, in a considerable number of individuals, a variation may be found that has been described as a “shift to the left.” In such cases fibers from the left crus of the diaphragm enter into formation of the right side of the esophageal hiatus. In some instances the muscles to the right of the esophageal hiatus may take origin entirely from the left crus and those to the left of the hiatus entirely from the right crus. The suspensory muscle of the duodenum (suspensory ligament of the duodenum, ligament of Treitz) typically originates from the fibers of the right crus of the diaphragm that pass to the right of the esophagus.

Histology of Esophagus

The esophagus, like other parts of the gastrointestinal tract, is made up of a mucosa, a submucosa, a muscularis externa, and an adventitia. The mucosa is subdivided into epithelium, lamina propria, and muscularis mucosae. The epithelium that lines the lumen of the esophagus is stratified squamous epithelium, which is continuous with the epithelial lining of the pharynx. The surface cells of this epithelium are flattened and contain a few keratohyaline granules, but do not form a keratinized layer. An abrupt transition takes place between the stratified squamous epithelium of the esophagus and the simple columnar epithelium of the stomach along an irregular zigzag line, known as the Z line, situated slightly superior to the cardiac region of the stomach. The lamina propria is a layer of loose connective tissue deep to the epithelium. The muscularis mucosae is a thin layer of smooth muscle deep to the lamina propria and is continuous with the pharyngeal aponeurosis. A transition from connective tissue to muscular tissue takes place in this aponeurosis at about the level of the cricoid cartilage. It contains both longitudinal smooth muscle fibers and some elastic tissue and is thicker at the lower end of the esophagus.

The submucosa is formed by dense irregular connective tissue and contains both elastic and type I collagen fibers, as well as blood vessels and nerves supplying the mucosa. Lymphocytes are scattered in moderate numbers through both the lamina propria and the submucosa, and occasionally these may be found in isolated concentric groups. In its contracted state, the esophageal mucosa is thrown into irregular longitudinal folds. The submucosa extends into these folds, but the more superficial muscular layers do not.

The muscularis externa consists of an inner circular layer and an outer longitudinal layer. A thin layer of connective tissue exists between the two layers, in which is embedded the myenteric ganglia and plexus (of Auerbach). Between the muscularis externa and the submucosa are the submucosal ganglia and plexus (of Meissner) and several blood vessels. The musculature of the upper one fourth of the esophagus is generally striated in character, the second fourth contains both striated and smooth muscle, and the lower half is composed entirely of smooth muscle. The adventitia is a layer of loose connective tissue on the surface of the esophagus that anchors the organ to the surrounding structure.

Two types of glands can be recognized in the esophagus. The esophageal glands proper (of Brunner) are irregularly distributed throughout the entire length of the tube. They are small, compound mucous glands. Their ducts penetrate the muscularis mucosae and their branched tubules lie in the submucosa. They are somewhat more prominent in the superior part of the esophagus. The other type of glands is known as the esophageal cardiac glands because they closely resemble or are identical with the cardiac glands of the stomach. They are found just above the cardiac region of the stomach, in the distal esophagus. They are also occasionally found proximally, a few centimeters below the level of the cricopharyngeus muscle. They differ from the esophageal glands proper in that their ducts do not penetrate the muscularis mucosae and their branched and coiled tubules are located in the lamina propria, not in the submucosa.

Blood Supply of Esophagus

The blood supply of the esophagus is extremely varied. The cervical region of the esophagus receives blood from esophageal branches of the inferior thyroid artery. The majority of esophageal branches arise from the terminal branches of this artery; its ascending and descending portions frequently give rise to one or more esophageal branches. The esophageal branches on the anterior aspect of the esophagus give small branches to the nearby trachea. Accessory arteries to the cervical esophagus may arise from the subclavian, common carotid, vertebral, ascending pharyngeal, superficial cervical, and thyrocervical arterial trunk.

The thoracic segment of the esophagus is supplied by branches from the (1) bronchial arteries, (2) thoracic aorta, and (3) right intercostal arteries. The bronchial arteries give off esophageal branches at or below the tracheal bifurcation, contributions from the left inferior bronchial artery being the most common. Patterns of the bronchial arteries vary markedly. The standard textbook type (two left, one right) occurs only in about one half of persons. Aberrant types are one right and one left (25%), two right and two left (15%), one left and two right (8%), and, in some instances, three right or three left. Near the bifurcation point of the trachea, the esophagus may receive additional twigs from the abdominal aorta, aortic arch, and intercostal and internal thoracic arteries. The aortic branches to the thoracic esophagus are not segmentally arranged, nor are they four in number, as commonly taught, but are only two unpaired vessels. The superior esophageal branch of the thoracic aorta is short (3 to 4 cm) and usually arises at the level of T6 to T7. The inferior esophageal branch of the thoracic aorta is longer (6 to 7 cm) and arises at the T7 to T8 disk level. Both arteries pass posterior to the esophagus and divide into ascending and descending branches that anastomose longitudinally, with descending branches from the inferior thyroid artery as well as bronchial arteries and with ascending branches from the left gastric and left inferior phrenic arteries. Right intercostal arteries, mainly the fifth, give rise to esophageal branches in about 20% of the population.

The abdominal esophagus receives its blood supply primarily through branches that arise from the celiac trunk. The left gastric artery is one of the three typical branches of this trunk and is the major blood supply to the abdominal esophagus. An additional blood supply comes from the short gastric arteries and from the recurrent branch of the left inferior phrenic artery, given off by the latter after it has passed posterior to the esophagus in its course to the diaphragm. The left gastric artery supplies cardioesophageal branches, either via a single vessel that subdivides or via several branches (two to five), given off in seriation before its division into an anterior and a posterior primary gastric branch. Other arterial sources to the abdominal esophagus may be branches from (1) an aberrant left hepatic from the left gastric, an accessory left gastric artery from the left hepatic, or branches from a persistent primitive gastrohepatic arterial arc; (2) cardioesophageal branches from the splenic trunk, its superior polar, terminal divisions (short gastric arteries), and its occasional large posterior gastric artery; or (3) a direct, slender cardioesophageal branch from the aorta or celiac or first part of the splenic artery.

With every resection operation, areas of devascularization may be induced by (1) a resection of the cervical segment that is too low (the segment should always have a supply from the inferior thyroid); (2) excessive mobilization of the esophagus at the tracheal bifurcation and laceration of the bronchial arteries; or (3) excessive sacrifice of the left gastric and the recurrent branch of the left inferior phrenic to facilitate gastric mobilization. The anastomosis about the abdominal esophagus is usually very copious, but in some instances it may be extremely meager.

Venous Drainage of Esophagus

The venous drainage of the esophagus is effected by tributaries that empty into the azygos and hemiazygos systems. Drainage begins in a submucosal venous plexus, branches of which, after piercing the muscle layers, form a venous plexus on the external surface of the esophagus. Tributaries from the cervical esophageal veins drain into the inferior thyroid vein, which empties into the right or left brachiocephalic vein, or into both. Tributaries from the thoracic esophageal veins on the right side join the azygos, right brachiocephalic, and, occasionally, vertebral vein. On the left side they join the hemiazygos, accessory hemiazygos, left brachiocephalic, and, occasionally, vertebral vein. Venous tributaries from the abdominal esophagus drain mostly into the hepatic portal vein by way of the left gastric vein, and to a lesser degree, the short gastric veins. A small amount of venous blood from the abdominal esophagus may drain to the left inferior phrenic vein before joining the inferior vena cava directly or via the suprarenal and then left renal vein.

The composition and arrangement of the azygos system of veins are extremely variable. The azygos vein arises in the abdomen from the ascending right lumbar vein that receives the first and second lumbar and the subcostal veins. It may arise directly from the inferior vena cava or have connections with the right common iliac, or renal, vein. In the thorax it receives the right posterior intercostal veins from the fourth to the eleventh spaces and terminates by entering the right side of the superior vena cava. The highest intercostal vein from the first space drains into the right brachiocephalic or, occasionally, into the vertebral vein. The veins from the second and third spaces unite in a common trunk (right superior intercostal) that ends in the terminal arched portion of the azygos. The hemiazygos vein arises as a continuation of the left ascending lumbar or from the left renal vein. It receives the left subcostal vein and the intercostal veins from the 8th or 9th to the 11th spaces and then crosses the vertebral column posterior to the esophagus to join the azygos. The accessory hemiazygos vein receives the intercostal veins from the fourth to the seventh or eighth spaces and then crosses the spine posterior to the esophagus to join the hemiazygos or to end separately in the azygos. Above, it may communicate with the left superior intercostal that drains the second and third spaces and ends in the left brachiocephalic. Drainage of the first space is into the left brachiocephalic or vertebral vein.

Often the hemiazygos, accessory hemiazygos, and superior intercostal trunk form a continuous longitudinal venous channel, with no connections to the azygos vein on the right. On the other end of the spectrum, the azygos vein may be located along the anterior aspect of the vertebral body and veins from the left side of the thorax may drain directly to it and a hemiazygos or accessory hemiazygos vein are not formed. The left azygos system may be reduced to a slender channel, the main left venous drainage of the esophagus and the intercostal spaces then being in the veins of the respective vertebrae. Interruptions in the left azygos system by crossing to the right azygos usually occur between the seventh and ninth intercostal veins, the most common vertebral level of crossing being T8.

At the inferior end of the esophagus, branches from the left gastric vein are continuous with the lower esophageal branches. Portal hypertension may shunt blood into the lower esophageal branches and thereafter into the superior vena cava via the azygos and hemiazygos veins. These vessels may dilate, creating esophageal varicosities. From this same region, blood may be shunted into the splenic vein, retroperitoneal veins, and inferior phrenic vein of the diaphragm, reaching the caval system. Because short gastric veins pass up from the splenic to the cardioesophageal end of the stomach, thrombosis of the splenic vein may readily lead to esophageal varices and fatal hemorrhages.

Lymphatic Drainage of Esophagus

The esophagus contains a rich network of lymphatic vessels, largely in the lamina propria of the mucosa but also in the other layers.

From the cervical esophagus, lymph vessels course chiefly to the inferior deep cervical (internal jugular) lymph nodes and possibly also to the nearby paratracheal nodes situated in the groove between the esophagus and trachea. The internal jugular lymph nodes, a subdivision of the deep cervical nodes, lie along the internal jugular vein, stretching from the parotid gland to the clavicle. On the left side, they drain to the thoracic duct and on the right to the short right lymph duct, which opens into the right subclavian vein at the angle formed by the latter with the internal jugular vein.

From the thoracic esophagus, lymphatic fluid drains posteriorly to the posterior mediastinal and intercostal lymph nodes. The posterior parietal nodes are formed of the posterior mediastinal and intercostal nodes. The posterior mediastinal nodes lie alongside the vertebral column, and the intercostal nodes are in the nearby intercostal spaces. Both these groups drain generally superiorly and eventually empty into the thoracic duct or into the right lymph duct, which terminates at the right subclavian vein, where it joins the right jugular vein. The superior phrenic nodes near the posterior esophagus are closely associated with the posterior parietal nodes, to which they drain. Anteriorly, the thoracic esophagus drains to the paratracheal, superior tracheobronchial, and inferior tracheobronchial lymph nodes; more inferiorly, lymphatic fluid drains to juxtaesophageal and superior phrenic lymph nodes before flowing superiorly.

The paratracheal nodes form a chain on each side alongside the trachea along the course of the recurrent nerves. The superior and inferior tracheobronchial nodes are the group that is situated about the bifurcation of the trachea and in the angle formed by the bifurcation. These lymph nodes may be responsible for the formation of traction diverticula when they become fibrosed as a result usually of tuberculous involvement. The tracheal and tracheobronchial nodes drain superiorly and usually form on each side a bronchomediastinal trunk, which, in turn, joins either the thoracic duct or the right lymph duct. They may, however, also have independent openings into the veins or may unite with the internal thoracic chain or a low node of the internal jugular chain.

From the short abdominal portion of the esophagus, lymphatic drainage is similar to that from the upper portion of the lesser curvature of the stomach, chiefly to the cardiac nodes of the stomach, which are a subdivision of the left gastric lymph node group. From here, in turn, drainage is to the celiac lymph nodes. Some lymph vessels from this region also pass superiorly through the esophageal hiatus of the diaphragm and connect with the vessels and nodes above the diaphragm. Drainage from the left gastric nodes is along the course of the left gastric artery and coronary vein to the celiac nodes situated on the aorta in relation to the root of the celiac trunk. These nodes, in turn, empty into the cisterna chyli or the thoracic duct.

Innervation of Esophagus

The esophagus is innervated by the vagus nerve as well as several sympathetic nerves. The vagus nerve innervates the glands and muscles of the esophagus, and the sympathetic input primarily innervates the capillary sphincters associated with the organ's blood supply.

Intrinsic Innervation of Alimentary Tract

From the esophagus to the rectum, the intrinsic innervation of the alimentary tract is effected through the enteric nervous system. This network is composed of numerous ganglion cells interconnected by their axons and dendrites. They are found primarily in two locations, between the longitudinal and circular layers of the muscularis externa, as the myenteric (Auerbach) plexus, and between the muscularis externa and the submucosa, as the submucosal (Meissner) plexus. The former is relatively coarse, and its meshes consist of thick, medium, and thin bundles of fibers, which are described as its primary, secondary, and tertiary parts. The submucosal plexus is more delicate. Other subsidiary plexuses have been described, such as a rarefied subserous plexus in those parts of the alimentary canal covered by peritoneum, but minute details of these need not be given.

The enteric plexuses vary in pattern and density in different parts of the alimentary tract and in different species of animals. They are well developed in the regions from the stomach to the lower end of the rectum and are less well formed in the esophagus, particularly in its upper half, which is primarily skeletal muscle. The ganglion cells are also not distributed uniformly; thus, the density of cell distribution in the myenteric plexus is lowest in the esophagus, rises steeply in the stomach until it reaches its peak at the pylorus, falls to an intermediate level throughout the small intestine, and then increases again along the colon and especially in the rectum. The density of cell population in the submucosal plexus seems to run roughly parallel to that in the myenteric plexus.

The enteric plexuses contain postsynaptic sympathetic axons, as well as presynaptic and postsynaptic parasympathetic axons. These exist alongside afferent axons from the alimentary tract, and the intrinsic ganglia of each plexus. Vagal presynaptic fibers form synapses with the ganglion cells whose axons are the postsynaptic parasympathetic fibers. The sympathetic presynaptic fibers have already relayed in paravertebral or prevertebral ganglia, and so the sympathetic fibers in the plexuses pass through to their terminations without synaptic interruptions. The afferent fibers from the esophagus, stomach, and duodenum are carried to the brainstem and cord through the vagal and sympathetic nerves supplying these parts, but they form no synaptic connections with the ganglion cells in the enteric plexuses.

Two chief forms of nerve cells, types I and II, occur in the enteric plexus. Type I cells are multipolar and confined to the myenteric plexus, and their dendrites branch close to the parent cells. Their axons run for varying distances through the plexuses to establish synapses with cells of type II, which are more numerous and are found in both myenteric and submucosal plexuses. Most type II cells are multipolar, and their longer dendrites proceed in bundles for variable distances before ramifying in other cell clusters. Many of their axons pass outward to end in the muscle coats, and others proceed inward to supply the muscularis mucosae and to ramify around vessels and between epithelial secretory cells; their distribution suggests that they are motor or secretomotor in nature.

Another group of cells in the area are the interstitial cells of Cajal, which are associated with the terminal networks of all autonomic nerves and act as pacemaker cells of the smooth muscle layers of the gastrointestinal tract. The frequency of this pacemaker activity varies between different organs. Under experimental conditions peristaltic movements occur in isolated portions of the gut, indicating the importance of the intrinsic neuromuscular mechanism, but the parasympathetic and sympathetic nerves regulate the activity of the gut tube. Local reflex arcs may exist in the enteric plexuses; this possibility is supported by the observation that in addition to type I and II multipolar cells, much smaller numbers of pseudounipolar and bipolar cells can be detected in the submucosa, potentially acting as the afferent links in local reflex arcs.

In congenital megacolon (Hirschsprung disease) the enteric plexuses are apparently undeveloped or degenerated over a segment of the alimentary tract, although the extrinsic nerves are intact. The affected segment is tonically contracted and peristaltic movements are defective or absent. This results in distention of the proximal region of the gastrointestinal tract.